The present invention relates to high speed data transmission in digital mobile communication networks.
In mobile telecommunication systems of the time division multiple access (TDMA) type, time-division communication on the radio path takes place in successive TDMA frames, each of which consists of several time slots. In each time slot, a short information packet is sent as a radio frequency burst which has a finite duration and which consists of a set of modulated bits. The time slots are mainly used for transmitting control channels and traffic channels. On the traffic channels, speech and data are transmitted. On the control channels, signalling between a base station and mobile subscriber stations is carried out. An example of a TDMA radio system is the Pan-European mobile communication system GSM (Global System for Mobile Communications).
Other multiple access methods include code division multiple access (CDMA) and frequency division multiple access (FDMA). In a CDMA system, the channels are identified by spreading codes, and in FDMA systems each channel has a dedicated carrier frequency.
For communication in conventional mobile communication systems, each mobile station is assigned one traffic channel for data or speech transmission (single channel access). Thus, the GSM system, for instance, may have up to eight parallel connections to different mobile stations on a same carrier wave. The maximum data transmission rate on one traffic channel is restricted to a relatively low level according to the available bandwidth and the channel coding and error correction used in the transmission, for example in the GSM system to 9.6 kbit/s or 12 kbit/s. In the GSM system, a so-called half-rate (max. 4.8 kbit/s) traffic channel may be used for low rate data coding .
A high speed circuit switched data service HSCSD is currently being defined for digital mobile communication systems. HSCSD is based on parallel use of more than one traffic channel for a single data call. A high speed data signal is divided at the transmitting end into the parallel traffic channels (subchannels) for the duration of the transfer, and combined at the receiving end. In this manner it is possible to offer data transmission services in which the transmission rate is, depending on the number of allocated traffic channels, up to eight times the usual transfer rate. In the GSM system, for example, two parallel traffic channels will provide a transfer rate of 2xc3x979.6 kbit/s, which is enough for a 14.4 kbit/s modem or a facsimile terminal. Six parallel traffic channels enable a transfer rate as high as 64 kbit/s. Such multichannel data transmission is described e.g. in WO95131878.
However, the use of parallel traffic channels has produced a problem concerning how to divide the data stream between parallel traffic channels and how to synchronize the combining of the data received from the parallel traffic channels. The above is caused by the fact that the data units, such as data frames, may be received from the subchannels in a different order than in which they were transmitted because the transmission delays on the different subchannels are not necessarily equal. For the receiving party to be able to restore the correct sequence of the data received from the subchannels, the data units must be numbered. One of such numbering mechanisms is disclosed in WO96/18248.
It is characteristic to the HSCSD solutions being developed that the data units transferred in the subchannels have a fixed length. For example, in a non-transparent service of the GSM system the data unit is a 240-bit RLP frame according to the radio link protocol (RLP). In a transparent service of the GSM system, the data unit is a stripped ITU-T V.110 frame. The user data, e.g. facsimile codes, are packed in the information field of the RLP frames, or in bits reserved for user data in the V.110 frame. In the non-transparent case, the receiving party organizes the data received from the subchannels into the correct order on the basis of the RLP frame numbers. In the transparent case, numbering related to the data units and/or subchannels is used. Transfer of fixed length data units over a three-channel transmission link (ch1, ch2, ch3) using frame numbering 1-6 is shown by FIG. 4.
In some cases it is necessary to optimize the data units so as to utilize the speed of the radio interface as efficiently as possible for transfer of user data. For example, a transparent service in the GSM system may have to abandon the V.110 frame structure for the sake of higher user data rate. This means that the statuses of the terminal equipment interface and the HSCSD control information (e.g. numbering of data units) has to be transferred in redundant parts of the user data. This easily results in that the data units transferred in the subchannels vary in length. Transfer of variable length data units over a three-channel transmission link is shown in FIG. 5.
A problem encountered here is that while transferring a maximum length data unit via one subchannel there is time to transfer a large number of short data units via the other subchannels. For the receiving party to be able to restore the correct order of the data units received from the subchannels, the number space used for numbering the data units (numbering bits) must be large enough. A drawback here is that as the number of numbering bits increases the number of user data bits decreases; in other words, the efficiency of the transfer is impaired.
Short data units also cause other problems. If the transmission delay of parallel subchannels is unequal and the data units are short in comparison with the transmission delay tolerance (e.g. if the data unit has a length smaller than twice the transmission delay tolerance), the receiving party""s ability to restore the correct order of the data units has to be ensured by expanding the number space of the data units. Again, this deteriorates the transfer efficiency.
It is the object of the invention to improve the efficiency of data transmission when transmitting variable length or short data units through parallel traffic channels.
The invention relates to a method for high speed data transmission in a digital mobile communication system, the method comprising the following steps:
allocating at least two parallel traffic channels to a mobile station,
providing the data units with a sequence numbering at the transmitting end,
transmitting the data units provided with the sequence numbering to the receiving end via the allocated traffic channels,
restoring, at the receiving end, the sequence of the data units to match said sequence numbering. The method according to the invention is characterized by
concatenating short data units in data blocks,
transmitting each data block as a whole via the traffic channel.
In the invention, successive short data units are concatenated to form data blocks that are transmitted as a whole (without splitting) via a subchannel when the data units are of a variable length or when the data units are short in comparison with the maximum difference between transmission delays of parallel traffic channels. In case the data units vary in length, the long ones are transmitted without concatenation, provided with a dedicated data unit number. The data block advantageously has a length of approximately the same magnitude as the long data units so that the entities being transferred would have the same length as closely as possibly. On the other hand, from the point of view of transmission delays, it is advantageous to build the data blocks as long as possible to eliminate or minimize the effects on the number space of the difference between the transmission delays on the parallel subchannels. The number space required is also decreased by shortening the code word used for the numbering. This consequently improves the efficiency as the number of overhead bits transferred decreases.
In the preferred embodiment of the invention the data block is assigned a single data unit number. The receiving party may identify the borders between data blocks for example by the first data unit in each data block containing the data unit number of the data block. In case the data block contains successive data units, the data units will automatically be positioned in the correct sequence at the receiving end. The number space required will decrease and hence the code word employed for numbering is further shortened. As the data unit number is transferred in a concatenated data block just once, and not separately in each data unit of the block, the number of overhead bits transferred further decreases.
In the second embodiment of the invention, each data unit in a data block contains a data unit number. This results in that it is not strictly compulsory to concatenate successive data units into the data block. However, the number of bits to be transferred in this embodiment is higher than in the previous embodiment.